This invention concerns a device for determining the instant of reception of an acoustic wave received at a sensor intermittently connected to a recording or a processing system.
More particularly, the device is adapted to determine the instant of reception of acoustic waves directly received from a seismic source after triggering thereof, by means of a sensor arranged in a seismic streamer comprising a system for the sequential transmission of the information received at each of the sensors or sensor groups thereof.
The recording of the instant of arrival of the first acoustic wave received at the sensor of the seismic streamer, after the triggering of the seismic source, hereinafter designated as the reference instant of time, allows, by means of a comparison, to the determination of the propagation time of the echoes received after the reference time and, consequently, the depth of the subterranean layers producing these echoes can be determined.
In the reception systems of the prior art type, the reference time is determined by making use of a special sensor generally located at the head of the seismic streamer and permanently connected to the signal recording or processing system arranged on a ship, in the case of marine seismic prospecting, by means of a high-pass filter, i.e., an element adapted to detect the envelope of the filtered signal and a Schmitt trigger adapted to be triggered when the level of the detected signal is higher than an adjustable threshold value. The reference time is known with accuracy due to the permanent connection between the sensor and the recording or the processing system. This permanent connection of the special sensor is not only present in the conventional prior art seismic streamers where each sensor or sensor group is connected through conductors to the recording or processing system, but also in the seismic streamers where the transmission of information is achieved through a sequential transmission system called multiplexing system, and is provided in order to avoid an unacceptable inaccuracy in the detection of the reference instant, a disadvantage inherent in the very principle of the discontinuous connection mode of operation which characterizes multiplexing transmission, as it will become clearly apparent from the following description.